The present invention relates to a homogeneous substitution method of a cake using a filter press and an apparatus therefor, which are widely utilized as solid-liquid separation and which are applicable to many fields and, more particularly, to collection of effective materials contained in the cake and removal of impurities in the cake in various steps of separation of inorganic medicines, organic medicines, plastic materials and the like in a chemical industry, separation of activated carbon used in a decoloring step in a food industry, and filtering of a sludge in a hydrometallurgy, and so on.
Generally, a filter press has widely been utilized in various fields of industry. However, the filter press has the following drawbacks. That is, in the case where impurities contained in a cake are removed or effective ingredients contained in the cake are collected by the use of the filter press, cracks are formed in the cake or a washing liquid is not homogeneously dispersed in the cake in view of construction in operation from hydration to washing. Thus, the filter press is deteriorate in efficiency of washing and is said to be unsuitable. For this reason, in operation which requires precision washing, filtering apparatuses of another type have been used such as, for example, a horizontal-belt type filtering apparatus, a drum type filtering apparatus, and the like. Since, however, the conventional filtering apparatuses such as the horizontal-belt type filtering apparatus, the drum type filtering apparatus and so on are systems in which vacuum suction is chiefly utilized, filtering speed is slow. Accordingly, in the case where a plenty amount of cakes is processed or a material difficult in filtering is processed, an amount of investment with respect to the filtering equipment or installation is large so that the filtering apparatuses are not economical. In view of this, it has been desired to develop a novel filtering apparatus which is high in processing ability.
By the way, conventionally, various attempts have been made to efficiently wash the cake by the use of the filter press. However, these attempts have not been succeeded. These attempts will be revealed as follows.
(1) There is a case where a both-side filtering apparatus having a single-side squeezing mechanism or a diaphragm is used, and washing water enters a space between a squeezed filtering cloth and a squeezed filtering plate unit from a rear side of the squeezed filtering cloth under such a condition that the squeezed filtering cloth is squeezed by the diaphragm, to wash the space. In this case, since a plurality of filtering-cloth supports on the squeezed filtering plate unit are in intimate or close contact with a cake through the squeezed filtering cloth, the washing water is not homogeneously in contact with the cake. Accordingly, washing effect is low.
(2) There is a case where the washing water enters the space between a squeezing filtering plate unit and a squeezing filtering cloth from a rear side thereof under such a condition that the squeezing filtering cloth is not squeezed by the diaphragm. In this case, since filtering-cloth retainers on the diaphragm are in close contact with the cake through the squeezing filtering cloth, the washing water cannot homogeneously be distributed, similarly to the above (1), so that washing effect is low.
(3) There is a case where, after squeezing due to the diaphragm, pressurizing water on the diaphragm is released in pressure to wash the cake between the squeezing and squeezed filtering cloths, from the rear side of the squeezing filtering cloth. In this case, since the impact is applied to the cake at releasing of the pressure so that cracks are formed in the cake, the washing water flows in a short-circuit manner so that washing effect is reduced.
(4) There is a case where washing is effected from the rear side of the squeezing filtering cloth under such a condition that a cake is formed before squeezing or under a slurry condition as it is. In this case, no cracks occur in the cake. Since, however, liquid is rich with respect to a solid material, a plenty of washing water is required.
In order to solve the above-discussed problems, the inventors of this application have made various researches and experiments. As a result, it has been found that, if the following items (a) through (c) are satisfied, the cake can effectively be washed even in the filter press.
(a) The cake must be washed under such a condition that the cake is squeezed. That is, deformation must not occur in the cake at washing.
(b) Such operations must not be done that a condition of the compressed cake is not temporarily loosened or relaxed or an external force is not mechanically applied to the compressed cake.
(c) The washing water must be in direct contact with the cake from the rear side of the filtering cloth. Efficiency is low if a plurality of projections or filtering-cloth supports are in direct contact with the filtering cloth to squeeze the cake.